fix: default to custom for empty transaction and span types

[kruskall]

According to the spec:

If no transaction.type or span.type is provided or the value
is an empty string, the agent needs to set a default value
custom.

Add test to assert the behaviour is compliant to the spec.

Closes #1211

[apmmachine]

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• Start Time: 2022-07-27T07:21:27.909+0000

• Duration: 48 min 55 sec

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Test	Results
Failed	0
Passed	7475
Skipped	174
Total	7649

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[axw]

Looks good, but the module/apmot tests are failing, as they are depending on Type to be empty after construction.

Maybe there's a better alternative, but we I think we may need to set this default value when ending the transaction or span, if the value is empty. That's a bit more foolproof anyway, since users could directly set Type = "" after starting the transaction or span.

[apmmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (59/59)	💚
Files	99.346% (152/153)	👍
Classes	96.264% (335/348)	👍
Methods	90.274% (956/1059)	👍
Lines	82.111% (11094/13511)	👎 -0.029
Conditionals	100.0% (0/0)	💚

[axw]

LGTM!

[ugosan]

this should also fix this one elastic/kibana#126805